Biology subjects

Wilhelm, S.

Publications and source records attributed to Wilhelm, S..

6 recordsLinked to original sources

bioRxiv2024

Time of day of infection shapes development of a eukaryotic algal-Nucleocytoviricota virocell

Aureococcus anophagefferens represents one component of a model host-virus system (with the "giant virus" Kratosvirus quantuckense). Studies to define its ribocell (uninfected cells) and virocell (virus-infected cells) forms are needed, as both are abundant during algal blooms. A linkage between light-derived energy, virus particle production and virocell formation has been noted. We explored how the time of day (morning, afternoon, late day) of virus-host contact shaped virocell ontogeny. In parallel, we explored the need for light derived energy in this mixotrophic plankter by inhibiting photosystem II (PSII). Using flow cytometry and photochemical assessments, we examined the physiology of infected cells and controls, and estimated virus particle production by virocells. We observed distinct differences between ribocell and virocell response to treatments, including reductions in virus particle production during reduced light (i.e., duration) and PSII inhibition. Collectively this work demonstrates the importance of light in shaping the fate of infected cells and provides insight into the factors that constrain in situ blooms. Most significantly, we show that time of day when a virus and host come into contact influences viral particle production, and therefore bloom dynamics; a factor that needs to be considered in future bloom modeling work.

microbiology↗

bioRxiv2024

Metatranscriptomics reveals declines in ice cover influence winter viral community activity

Freshwater lakes are sentinels of environmental change, and climate change-driven declines in ice cover have been shown to disrupt aquatic communities and jeopardize ecosystem services. Viruses shape microbial communities and regulate biogeochemical cycles by acting as top-down controls, yet there is relatively little known about how declining ice cover will influence viral community activity. Lake Erie is a critical freshwater ecosystem and serves as a model system to assess how ice cover extent will affect winter limnology. We surveyed size selected surface water metatranscriptomes for conserved viral hallmark genes as a proxy for active virus populations and compared activity profiles between ice-covered and ice-free conditions from two contrasting winters. Active virus communities were present in both conditions, spanning diverse phylogenetic clades of bacteriophage (Caudovirales), giant viruses (Nucleocytoviricota), and RNA viruses (Orthornavirae). However, viral activity was significantly shaped by the extent of ice cover. Notably, viral richness and relative transcript abundance in the surface waters were reduced under ice relative to the ice-free conditions. Correlations with microbial community metrics suggest the differences in viral communities are at least in part driven by the decreased winter diatom bloom associated with declines in ice cover. Overall, our data suggest viral community activity is influenced by ice cover extent, and viruses may serve as sentinels of environmental disturbance and ecosystem response(s) to climate change. IMPORTANCEAs ice cover is projected to become increasingly rare on large temperate lakes, there is a need to understand how microbial communities during winter months might respond to these changing ice-cover conditions. Despite the documented controls viruses have on microbial communities, little is known regarding the relationship between virus activity and ice cover extent. By using metatranscriptomics to investigate virus communities, we show that viral community activity is sensitive to ice cover extent, likely due in part to ice cover-driven shifts in host community structure. This work serves to build our understanding of how viral communities will function in a future, potentially ice-free, climate.

microbiology↗

bioRxiv2024

Diel Partitioning in Microbial Phosphorus Acquisition in the Sargasso Sea

The daily cycle of photosynthetic primary production at the base of marine food webs is often limited by the availability of scarce nutrients. According to temporal niche partitioning theory, competition for scarce resources can be alleviated insofar as the intensity of nutrient uptake and assimilation activities are distributed heterogeneously across organisms over periodic input cycles. Recent analysis of community transcriptional dynamics in the nitrogen-limited subtropical North Pacific gyre revealed evidence of temporal partitioning of nitrogen uptake and assimilation between eukaryotic phytoplankton, cyanobacteria, and heterotrophic bacteria over day-night cycles. Here, we present results from a Lagrangian metatranscriptomic time series survey in the Sargasso Sea and demonstrate temporally partitioned phosphorus uptake in this phosphorus-limited environment. In the Sargasso, heterotrophic bacteria, eukaryotic phytoplankton, and cyanobacteria express genes for phosphorus assimilation during the morning, day, and dusk, respectively. These results support the generality of temporal niche partitioning as an emergent mechanism structuring uptake of limiting nutrients and facilitating coexistence of diverse microbes in open ocean ecosystems.

ecology↗

bioRxiv2024

Featural and spatial interference with functionally active and passive items in working memory

Functionally active and passive states in working memory have been related to different neural mechanisms. Memoranda in active states might be maintained by persistent neural firing, whereas memoranda in passive states might be maintained through short-term synaptic plasticity. We reasoned that this might make these items differentially susceptible to interference during maintenance, in particular that passively maintained items might be more robust. To test this hypothesis, we gave our participants a working memory task in which one item was prioritised (active) by always probing it first, while the other item was deprioritised (passive) by always probing it second. In two experiments, on half the trials, we presented an interfering task during memory maintenance, in which the stimuli matched either the feature dimension of the memory items (colour or orientation), or their spatial location. Whether the interfering task appeared on a given trial was unpredictable. In a third experiment where participants were given prior knowledge of the interference condition, and finally in a fourth experiment we used a reward-based prioritisation cue. Across experiments, we found that both active and passive memory items were affected by interference to a similar extent, with overall performance being closely matched in all experiments. We further investigated precision and probability of target response parameters from the standard mixture model, which also showed no differences between states. We conclude that active and passive items, although potentially stored in different neuronal states, do not show differential susceptibility to interference. Public significance statementThe ability to briefly remember information is critical to human cognition. Our so-called working memory is nevertheless rather limited, able to hold only a few items at any one time, and prone to forgetting when we are briefly distracted. Yet, there is reason to believe that not all information in working memory is equally vulnerable. Items that are more passively stored, because they will only be required after some time, might be more resilient to interference. Items that are stored actively, for more immediate recall, might be more easily disrupted. Here, we investigated the effect of an interference task on the retention of both active and passive items in working memory. Our results showed that active and passive items are equally affected by interference, suggesting that resilience in working memory does not depend on the functional state of the items therein.

neuroscience↗

bioRxiv2023

Spatially and Functionally Distinct mTORC1 Entities Orchestrate the Cellular Response to Amino Acid Availability

Amino acid (AA) availability is a robust determinant of cell growth, through controlling mTORC1 activity 1. According to the predominant model in the field, AA sufficiency drives the recruitment and activation of mTORC1 on the lysosomal surface by the heterodimeric Rag GTPases, from where it coordinates the majority of cellular processes (reviewed in 2,3). Importantly, however, 15 years after its initial discovery, the teleonomy of the proposed lysosomal regulation of mTORC1, and where mTORC1 acts on its effector proteins remain enigmatic 4. Here, by using multiple pharmacological and genetic means to perturb the lysosomal AA sensing and protein recycling machineries, we describe the spatial separation of mTORC1 regulation and downstream functions in mammalian cells, with lysosomal and non-lysosomal mTORC1 phosphorylating distinct substrates in response to different AA sources. Moreover, we reveal that a fraction of mTOR localizes at lysosomes due to basal lysosomal proteolysis that locally supplies new AAs, even in cells grown in the presence of extracellular nutrients, whereas cytoplasmic mTORC1 is regulated by exogenous AAs. Overall, our study substantially expands our knowledge about the topology of mTORC1 regulation by AAs, and hints at the existence of distinct, Rag- and lysosome-independent mechanisms that control its activity at other subcellular locations. Given the importance of mTORC1 signalling and AA sensing for human ageing and disease 2, our findings will likely open new directions toward the identification of function-specific mTORC1 regulators, and suggest new targets for drug discovery against conditions with dysregulated mTORC1 activity in the future.

cell biology↗

bioRxiv2023

Maintenance of colour memoranda in activity-quiescent working memory states: Evidence from impulse perturbation

The neural mechanisms underlying working memory maintenance pose a challenge for investigation, as sustained neural activity may not always be observable. To address this, the method of impulse perturbation has been employed to examine memorized information during activity-quiescent periods. However, this approach has mainly focused on spatially localized or referenced stimuli, leaving it unclear whether non-spatial memoranda share similar neural maintenance mechanisms. This study aimed to fill this gap by applying the impulse perturbation method to working memory for colours, which are inherently non-spatial stimuli. EEG data from 30 participants performing a delayed match-to-sample task were analysed, with one of the presented items being retro-cued as task-relevant. Our findings indicate that both cued and uncued colours could be decoded from impulse-evoked activity, in contrast to previous reports on working memory for orientation gratings. Additionally, we explored colour decoding from ongoing oscillations in the alpha band and discovered that cued items could be decoded, potentially influenced by attention, whereas uncued items could not. These results suggest subtle differences between the representation of colours and stimuli with spatial properties. However, they also demonstrate that both types of information can be accessed through visual impulse perturbation, regardless of their specific neural states.

neuroscience↗